Vacuum insulation materials are thermal insulation materials in which a core is vacuum-packaged with a gas-barrier packaging material and the interior is maintained in a vacuum state, to elicit thereby the effect of lowering the thermal conductivity of a gas. Vacuum insulation materials have excellent thermal insulation properties, and hence are used in appliances such as freezers, refrigerators, cooler/warmer boxes and vending machines, in the production of residential wall materials and in other various fields.
As the core of the vacuum insulation material there are used inorganic fiber cores of glass fiber or the like, resin-foam cores of urethane foam or the like, and fine-powder cores of micro-powdered silica or the like. Vacuum insulation materials that utilize inorganic fiber cores, in particular, exhibit excellent thermal insulation properties, and are therefore widely used in applications where yet higher thermal insulation properties are required. Inorganic fiber cores are improved in various ways in order to further enhance the thermal insulation properties of the vacuum insulation material.
When the orientation state of the inorganic fibers that make up the core is irregular, for instance, the thermal conductivity of the core increases, whereupon the core inorganic fibers themselves constitute heat transfer paths. In Patent Document 1 (identified further on), accordingly, thermal conduction by the inorganic fibers is curtailed, and the thermal conductivity of the core is further reduced, by stacking and arranging the inorganic fibers substantially perpendicularly to the heat transfer direction.
When a binder is used to mold the inorganic fiber core, heat conduction in the heat insulation direction increases, on account of thermal cross-linking of the solidified binder. Such being the case, Patent Document 2 (identified further on) discloses the feature of producing a core that is molded into a predetermined shape without using any binder.
Patent Document 1: Japanese Patent Publication No. 3513143
Patent Document 2: Japanese Patent Publication No. 3580315 B
In vacuum insulation materials that utilize inorganic fiber cores, however, thermal conductivity rose significantly accompanying rises in atmosphere temperature, and thermal insulation properties failed to be sufficiently satisfied in high-temperature environments. The above may be attributed to the fact that, since mobility of gas molecules in inorganic fiber cores is comparatively good, the motion of the gas molecules becomes more vigorous as the atmosphere temperature rises, and the thermal conductivity increases accordingly.
When the vacuum insulation material was exposed to a high-temperature environment, moreover, outgas was released by the packaging material and so forth, the degree of vacuum dropped gradually, and thermal insulation properties degraded readily over time.